I have a Hayward H400 heater, I think ED1. It's the one with no digital display and a manual thermostat knob (it only has this one knob). I have not used it this season and have had no leaks this season, but when I turned it on today, I heard something that sounds like a drop of water falling on something hot and sizzling. It happens about every 20 seconds or so, but again, when the heater is off I see no signs of leakage anywhere. This is only my second year with a pool and I don't remember hearing this last year. What could it be?